Recently, the performance and functionality of digital cameras and digital movie cameras that use some solid-state image sensor such as a CCD and a CMOS (which will be sometimes simply referred to herein as an “image sensor”) have been enhanced to an astonishing degree. In particular, the size of a pixel structure for use in an image sensor has been further reduced these days thanks to rapid development of semiconductor device processing technologies, thus getting an even greater number of pixels and drivers integrated together in an image sensor. As a result, the resolution of an image sensor has lately increased significantly from about one million pixels to ten million or more pixels in a matter of few years. Meanwhile, the greater the number of pixels in an image sensor, the lower the intensity of the light falling on a single pixel (which will be referred to herein as a “light intensity”) and the lower the sensitivity of the camera tends to be.
On top of that, in a normal color camera, a subtractive color filter that uses an organic pigment as a dye is arranged over each photosensing section of an image sensor, and therefore, the optical efficiency achieved is rather low. In a Bayer color filter, which uses a combination of one red (R) pixel, two green (G) pixels and one blue (B) pixel as a fundamental unit, each color filter transmits a light ray with only one color component and absorbs a light ray with any other color component. Specifically, the R filter transmits an R ray but absorbs G and B rays, the G filter transmits a G ray but absorbs R and B rays, and the B filter transmits a B ray but absorbs R and G rays. That is to say, each color filter transmits only one of the three colors of R, G and B and absorbs the other two colors. Consequently, the light ray that can be used is only approximately one third of the visible radiation falling on each color filter.
To overcome such a decreased sensitivity problem, Patent Document No. 1 discloses a method for increasing the quantity of the light received by attaching an array of micro lenses to a photodetector section of an image sensor. According to this technique, the incoming light is condensed with those micro lenses, thereby substantially increasing the optical aperture ratio. And this technique is now used in almost all solid-state image sensors. It is true that the aperture ratio can be increased substantially by this technique but the decrease in optical efficiency by color filters still persists.
Thus, to avoid the decrease in optical efficiency and the decrease in sensitivity at the same time, Patent Document No. 2 discloses an image sensor for taking in as much incoming light as possible by using multilayer filters and micro lenses in combination. Such an image sensor includes a combination of multilayer filters, each of which does not absorb light but selectively transmits only a component of light falling within a particular wavelength range and reflects the rest of the light falling within the other wavelength ranges. Each filter makes only a required component of the incoming light selectively incident on its associated photosensing section and reflects the other light rays.
FIG. 7 is a schematic cross-sectional view of the image sensor disclosed in Patent Document No. 2. In the image sensor shown in FIG. 7, the light that has impinged on a condensing micro lens 11 has its luminous flux adjusted by an inner lens 12, and then enters a filter 13, which is designed to transmit an R ray but reflect rays of the other colors. The light ray that has been transmitted through the filter 13 is then incident on a photosensitive cell 23 that is located right under the filter 13. On the other hand, the light ray that has been reflected from the filter 13 enters the adjacent filter 14, which is designed to reflect a G ray but transmit rays of the other colors. The G ray that has been reflected from the filter 14 is incident on a photosensitive cell 24 that is located right under the filter 14. On the other hand, the B ray that has been transmitted through the filter 14 is reflected from the filter 15 and then incident on a photosensitive cell 25 that is located right under the filter 15. In this manner, the visible radiation that has impinged on the condensing micro lens 11 is not lost but their RGB components can be detected by the three photosensitive cells non-wastefully. Such an image sensor can certainly condense the incoming light effectively through the micro lenses but needs to have pixels in the three colors of RGB, thus requiring a high density device structure.
The principle of color separation disclosed in Patent Document No. 2 is the same as that of the two- and three-tube color cameras that use splitting the incoming light through the multilayer filters and total reflection at the interface as disclosed in Patent Documents Nos. 3 and 4, respectively. FIGS. 8A and 8B illustrate optical prisms for use in the two- and three-tube color cameras disclosed in Patent Documents Nos. 3 and 4, respectively. The prism 21 shown in FIG. 8A separates R and B rays from a G ray. On the other hand, the prism 22 shown in FIG. 8B separates the R, G and B rays from each other. In the multi-panel color cameras disclosed in Patent Documents Nos. 3 and 4, information about the respective colors is received on an image basis. On the other hand, in the device disclosed in Patent Document No. 2, information about the respective colors is received on a pixel basis. In any case, pixels in the three colors are needed.
Thus, to overcome such problems with the related art, Patent Document No. 5 discloses a technique for increasing the optical efficiency by using multilayer filters and reflected light, although some loss of the incoming light is involved. FIG. 9 is a partial cross-sectional view of an image sensor that adopts such a technique. As shown in FIG. 9, multilayer filters 32 and 33 are embedded in a light-transmitting resin 31. Specifically, the multilayer filter 32 transmits a G ray and reflects R and B rays, while the multilayer filter 33 transmits an R ray and reflects G and B rays.
Such a structure cannot receive a B ray at its photosensing section but can sense R and G rays entirely under the following principle. First, if an R ray impinges on the multilayer filters 32 and 33, the R ray is reflected from the multilayer filter 32, is totally reflected from the interface between the light-transmitting resin 31 and the air, and then strikes the multilayer filter 33. Then, almost all of the R ray that has impinged on the multilayer filter 33 will be incident on the photosensing section by way of the organic dye filter 35 and the micro lens 36 that transmit the R ray, even though only a part of the light is reflected from the metal layer 37. On the other hand, if a G ray impinges on the multilayer filters 32 and 33, the G ray is reflected from the multilayer filter 33, is totally reflected from the interface between the light-transmitting resin 31 and the air, and then strikes the multilayer filter 32. Then, almost all of the G ray that has impinged on the multilayer filter 32 will eventually be incident on the photosensing section with virtually no loss by way of the organic dye filter 34 and the micro lens 36 that transmit the G ray.
According to the technique disclosed in Patent Document No. 5, only one of the three colors of RGB is lost but light rays of the other two colors can be received with almost no loss based on the principle described above. That is why there is no need to provide photosensing sections for all of the three colors of RGB. In this case, compared to an image sensor that uses only organic dye filters, it can be seen that the optical efficiency can be doubled by this technique. Nevertheless, even if such a technique is adopted, one out of the three colors should still be sacrificed.
The problem described above is a problem with a color image capturing system. However, a color display system also has a similar problem. Since a color liquid crystal display device of today uses RGB organic dye filters, only approximately one third of the incoming light can be used and the other two thirds should be lost.